Intrinsic five-photon non-linear absorption of two-dimensional BN and its conversion to two-photon absorption in the presence of photo-induced defects

Abstract

Unlike most two-dimensional materials, h-BN nanoplatelets (BNNPs) exhibit multi-photon absorption. This was previously attributed to a two-photon absorption (2PA) process at 1.16 eV by Kumbhakar et al., Advanced Optical Materials, 3, 828, 2015. This is counter-intuitive because a 2PA process at 1.16 eV cannot excite electrons across the wide band gap of BNNPs (∼5.75 eV). Here, through systematic experimental and theoretical work, we provide compelling evidence that BNNPs exhibit five-photon absorption (5PA) at a low-laser fluence (<0.6 J/cm2). Furthermore, we show that a high laser fluence (>0.6 J/cm2) generates photo-induced defects in BNNPs that support a 2PA process. Accordingly, we assert that BNNPs inherently exhibit 5PA at a low laser fluence. In the previous study reported by Kumbhakar et al. a high laser fluence was used, which led to the observation of 2PA process.